It hits for 10-15k in raids for me, and is roughly 6-8% of a parse on trash when used. It's been perfectly fine since they stopped it from proccing for 50-60k.
It hasn't been completely broken. The issue has been that the buff that lets you cast Concerto didn't apply while you were casting a spell. If you have capped haste it ends while you're paused for your 5th or 6th auto attack after you use it.
EDIT: Also, the reuse time is fine.